Waxes are incorporated into polymer particles such as electrophotographic toners to aid their release from the fuser roller surfaces, without the need to use fuser oil, such as silicone oil, at the fuser. The domain sizes of these waxes affect the many properties of the toner, including the powder flow, tribocharging, release from the funer surface and glossing. The particle size of a wax dispersion may be controlled for example, in a separate step by milling the wax to form a dispersion of the wax in a fluid medium. For chemical preparation of toners, it is beneficial to mill the wax in a volatile organic solvent that is removed during the toner making process. While the wax domain size can be reduced during milling without a dispersing aid, it is usually desirable to have a milling aid that can act as a colloidal stabilizer for the wax particles. The stabilizer is intended to prevent agglomeration of the wax particles thereby increasing the efficiency of milling and reducing the viscosity of the dispersion, allowing for higher yields and consequently lower milling costs.
U.S. Pat. No. 5,756,244 discloses a method of incorporating wax into the toner at the compounding step, where all the ingredients of the toner and the wax are added to the equipment such as an extruder, two roll mill, kneader etc. Typically, the wax is dispersed in the molten state under the processing conditions without an added dispersant to ensure uniform, small wax domains. As a result, when the extruded matrix is pulverized the resulting particles may contain free wax particles or non-homogeneous distribution of wax in the particles, with a high content of surface wax. The surface wax and free wax can adversely affect the performance of the toner in the machine.
U.S. Pat. No. 7,026,087 discloses toner compositions made by melt-kneading toner materials including binder resin, wax, colorant, and a wax dispersant comprising a copolymer of an alpha-olefin, maleic anhydride and maleic anhydride monoester. As the components are melt-kneaded, there is no need for formation of a wax dispersion in an organic solvent in such a process.
It has been found that incorporating fine particles of wax into chemically prepared toners (CPT) results in toners that have little free wax and low surface wax compared to standard melt-pulverized toners. There are different ways of incorporating wax into toners made by chemical preparation. In U.S. Pat. No. 20040161687 A1, small wax particles obtained commercially are added to a solvent phase containing the rest of the toner materials. This organic phase is dispersed in an aqueous phase containing a water soluble surfactant and viscosity modifier to aid in the dispersion. However, the choice of waxes that are readily available with the desired small size is limited.
A way to control the domain size of the wax is to mill the wax within a solvent medium. U.S. Pat. Nos. 6,682,866; 6,800,412; and 6,824,945; and U.S. Patent application Ser. No. 20040115551 disclose combining a polyester binder, pigment, charge control agent and wax that are milled using zirconia beads. The organic phase is then dispersed, with controlled shearing, in an aqueous phase containing hydroxyapatite and sodium dodecyl benzene sulfonate as dispersants. The solvent is then removed by evaporation. While this method is useful for creating small domain sizes of the waxes, it is not cost effective to process the entire toner/wax/solvent mixture to reduce the domain size of the wax.
For CPTs made by the emulsion aggregation technology, aqueous dispersions of wax, latex, pigment and charge control agent are mixed in a reactor and aggregated to form toner-sized particles. Aqueous dispersions of wax can be prepared by several methods. U.S. Pat. Nos. 6,849,371 and 6,210,853 disclose the preparation of wax dispersions by using a sulfonated polyester as a dispersant, which is also the toner binder, raising the aqueous dispersion temperature to above the melting point of the wax, using a high pressure reactor and then emulsifying the wax. U.S. Pat. No. 6,808,851 discloses a similar method with an anionic surfactant as the stabilizer. U.S. Patent application No. 20040044108 A1 describes the details of preparing the wax dispersions. It is substantially more difficult to carry out the emulsion aggregation process and incorporate the wax, than by using a solvent to dissolve and disperse the toner components.
It is required that dispersions of waxes in solvents be prepared for incorporation into toners prepared by certain CPT processes. The dispersions are prepared by breaking down the wax particles into submicron particles using milling media in the presence of dispersants. Typically, dispersants that enable fast milling are good stabilizers. That is, they adsorb strongly to the wax particles, and provide a good barrier to particle flocculation. While flocculated dispersions can be milled, they are typically higher in viscosity and hard to handle in the process. Consequently, the particle concentrations in flocculated dispersions cannot be high. The desired dispersant and stabilizer should be able to produce fast milled dispersions with small particle size and low viscosity.
A method of manufacturing a wax dispersion that includes forming a solution of a solvent and a dispersant, the dispersant being a polymer of partially or fully hydrogenated styrene butadiene wherein the styrene mole fraction of the polymer is from 20 to 90, is disclosed in U.S. Patent application No. 20070299191 A1. Wax is then added to the solution to form a slurry. The particle size of the wax is reduced through comminution of the slurry. The wax dispersion is used for the manufacture of CPT particles. This method has the disadvantage that the dispersant used is not compatible with the binder resins such as for example, polyester binders, which can lead to flocculation of both the wax and the colorants such as pigment dispersions. When the wax dispersion is added to a resin binder, it would be desirable to have wax dispersants that not only produce fast milled dispersions with small particle size and low viscosity, but also are compatible with the binder resins to which they are added, in order to avoid flocculation. For example, pigment concentrates, or pigment pre-dispersions, are used to prepare toners employed in full color electrophotographic processes. Uniform dispersion of a colorant or pigment in a color toner is necessary for realizing the highest possible color gamut, minimizing the amount of toner laid down on a page, achieving reproducibility of color and electrostatic and physical properties, and optimizing the cost of toner preparation. The addition of various additives into the toner such as the wax, for example, can lead to toner with poor color quality and low tinctorial strength if the wax dispersion flocculates the pigment dispersion when the two come into contact during the toner making process.
It is an object of the present invention to provide a wax dispersion in a water-immiscible solvent that produces uniform small wax domains in a toner binder.
It is another object of the present invention to produce wax dispersions where the median particle diameter is less than 1 micrometer.
It is yet another object of the present invention to produce wax-containing polymer particles such as a toner derived from the improved wax dispersion of the invention, where the toner performance is improved.
These and other objects of the invention can be accomplished by use of wax dispersions prepared in water-immiscible solvents using a polymeric dispersant comprised of a copolymer that is compatible with both the wax and toner binders into which the wax may be dispersed.